1. Field of the Invention
This invention relates to surface-emitting single heterojunction laser/LED structures.
2. Description of Related Art
The importance of vertical laser emission for optoelectronic integration was recognized by, for example, K. Iga et al., 9th IEEE International Semiconductor Conference, 1984, Rio de Janeiro, Brazil, paper D3, p. 52. Vertical laser emission is particularly important in constructing large emitting areas which can be made to have narrow beam angles and high power outputs. Several types of surface-emitting laser are known. SpringThorpe et al., International Electron Devices Meeting, (1977) Washington, D.C., p. 571 disclose a standard double heterostructure cavity transverse to current flow. The cavity is electrically pumped over most of its length and two additional mirrors are used to divert the laser beam towards the device surface. K. Iga et al., Electronics Letters, 19, #13 (1983) p. 457 disclose a surface-emitting laser having a cavity perpendicular to the surface but pumped over a short length of the cavity by a pn junction co-planar with the surface.
Finally, Ito et al., Electronics Letters 20 #14 (1984) p. 577, elaborate on the Iga structure by elongating the cavity and introducing additional pumping along its length by a diffused homojunction.
The Ito et al structure, since it has no heterojunction between materials of different bandgap does not have effective carrier and light confinement in comparison with known planar light emitting diodes. Consequently, the laser threshold current of the device is relatively high and the laser slope efficiency is relatively low.